Global Positioning Systems (GPS) are based on satellite range measurements. In the urban Canyon and dense forest-type environments, due to high buildings, trees, and multi-path effects, there can be gaps in position determination. In application Ser. No. 08/115,087 an AM positioning system, based on range measurements using commercial AM radio broadcast stations, is used to provide a No-Gap positioning system. Both Am and FM (pilot tones or subcarrier) positioning systems using plural commercial broadcast radio stations are well known in the art. In contrast, the present invention is directed to a position or location determination system using conventional commercial television (TV) broadcast signals.
Television (TV) broadcast stations utilize a signal transmission format that permits the extraction of timing information from the signals and, utilizing the velocity of radio wave propagation, convert this timing information into distance information. With a sufficient number of measurements, each measurement made employing a signal from a television station located at a different location than the other stations, a geometric location solution can be made.
Television (TV) station signal utilization for location determination has numerous advantages which include:
a. Very high power is usually employed in metropolitan areas to insure good program coverage. The location system can therefore operate over the wide area covered by TV. Because of the frequency band of operation of the stations, substantial building penetration is also achieved by the signals. Because of the combination of high power and frequency of operation, the position or location determination system can operate within many structures (buildings, public structures, businesses, busses, trains, etc.) in the station coverage area. This is a distinct advantage over such systems as GPS, LORAN, OMEGA, and TRANSIT. For areas where there may be insufficient TV broadcast stations to provide location information, the invention may optionally incorporate a GPS location system and a logic circuit to select the most reliable of the location signals for use.
b. Very high power is usually employed by TV stations in metropolitan areas to deliver a quality picture and sound presentation to the user. Because these signals vary from instant to instant, are low in repetitive information content, and require high bandwidth at the receiver, high power is necessary to achieve coverage for the majority of the metropolitan population. The location system according to this invention uses the repetitive information (not the variable information) requiring a low bandwidth. Signals marginal for video purposes can be extremely useful for location purposes.
c. The signal format utilized by the television stations is uniform (specified by the FCC) permitting uniform measurement equipment and approaches in the U.S. The location measurement approach can in fact utilize technology employed in current portable TV receivers for the extraction of timing signals permitting a low cost receiver.
d. The signal format utilized by the TV stations is highly stable and accurate, permitting simple differential implementation of the location determination process. (If the signal formats were not stable and accurate, synchronized timing signals could be utilized to achieve location determination.)